1. Field of the Invention
The present invention relates to power amplifiers and communication devices. More particularly, the present invention relates to a power amplifier for transmission which is used in a radio communication device or the like requiring amplification with low distortion, and a communication device which includes the power amplifier.
2. Description of the Background Art
As in mobile communication systems for mobile phones or the like, radio communication systems using quasi-microwave and microwave bands have rapidly spread in recent years. That greatly contributes to attaining portable terminals of lighter weight and lower power consumption.
In order to achieve lighter weight of portable terminals, it is effective to use lighter batteries of a small capacity type. However, employment of the small capacity type generally shortens the time till the battery runs out. Therefore, lower power consumption, that is, improvement in the power efficiency is strongly required for power amplifiers for transmission which lead to most power consumption of terminals at the time of transmission.
In constant amplitude analog modulation/demodulation systems using conventional FM modulation/demodulation methods, power amplifiers can operate in a saturated state and thus they can be made more efficient relatively easily. Recently, however, communication systems which employ digital modulation/demodulation using QPSK (quadrature phase shift keying) modulation or the like with high frequency utilization efficiency have become the mainstream.
In the digital modulation/demodulation methods, information is transmitted by both the amplitudes and phases of signals, and thus power amplifiers are required to linearly amplify input signals. Generally, as an increase in output power due to the increased input power approaches saturation, the power amplifiers come to have higher distortion and power efficiency. Therefore, high power efficiency and low distortion are in a tradeoff relationship. Attempts to improve the power efficiency are often made by adding a distortion compensation circuit so that the operation is performed with low distortion even at high input power. One example is described in Japanese Patent Laying-Open No. 9-260964 entitled xe2x80x9cHigh Frequency Amplification Circuitxe2x80x9d (hereinafter, referred to as Document 1).
FIG. 15 shows a circuit configuration of a power amplifier disclosed in Document 1. Referring to FIG. 15, a power amplifier 900 includes a bipolar transistor Tr90 for power amplification, a diode D90, a capacitor C90, and bias resistors R91, R92. Diode D90 and capacitor C90 form a distortion compensation circuit. When a bias voltage Vb is supplied, the base bias condition of bipolar transistor Tr90 is determined by the direct current characteristics of bias resistors R91, R92 and diode D90.
Capacitor C90 has capacitance which is regarded as a grounded state regarding a high frequency at the operating frequency of power amplifier 900. Impedance measured from the base end of bipolar transistor Tr90 toward diode 90 is only the resistance and capacitance components of diode D90 with regard to a high frequency. The impedance is equivalent to parallel connection between the base and emitter of bipolar transistor Tr90 with regard to a high frequency.
An input signal causes the instantaneous voltage between the base and emitter of bipolar transistor Tr90 to fluctuate with time. However, since a diode characteristic is observed between the base and the emitter, higher voltage fluctuation and lower voltage fluctuation of the instantaneous voltage are asymmetrical, with the center being a voltage when a signal is not applied. Thus, the average voltage varies with input power. Specifically, because of the diode characteristics, when the voltage across the diode increases and the current increases, then the impedance is lowered. Since the voltage amplitude at the higher voltage side is small, the average voltage is shifted toward the lower voltage side because of the input signal. The shift amount increases with an increase in the input power.
The capacitance component of a diode depends on a voltage across the both ends of the diode (both end voltage dependence). Therefore, the voltage shift due to an increase in input power changes the capacitance between the base and emitter of bipolar transistor Tr90. Thus, the reactance component of bipolar transistor Tr90 from the base end is changed, which changes the transmit of a signal. This is xe2x80x9camplitude-phase distortionxe2x80x9d as a cause of distortion for a power amplifier.
In power amplifier 900 shown in FIG. 15, phase distortion caused by the non-linearity of capacitance between the base and emitter of bipolar transistor Tr90 is compensated for by adding a distortion compensation circuit formed of diode D90 and capacitor C90.
In other words, an increase in input power decreases the average voltage of the diode portion (between the base and emitter) of bipolar transistor Tr90 and, at the same time, increases the average voltage across the both ends of diode D90 which is connected in parallel, with regard to a high frequency, with the base and emitter of bipolar transistor Tr90. Therefore, the change in the diode capacitance value between the base and emitter of bipolar transistor Tr90 and the change in the capacitance value of diode D90, which are caused by an increased or decreased input power, offset each other, mitigating the dependence of the transmit on the input power in the power amplifier. Thus, bipolar transistor Tr90 can effectively maintain linearity even at input power closer to saturation. As a result, the power efficiency is improved.
If bias power supply Vb and the base of bipolar transistor Tr90 are connected only through fixed resistance, the higher the base power caused by an increase in input power, the higher the effect of suppressing an increase in the base current becomes, which is caused by a reduction in the voltage at the fixed resistance portion. Therefore, the collector current is prevented from increasing, which reduces a gain due to an increase in the input power, that is, causes amplitude-amplitude distortion. In power amplifier 900, as the base current flowing in diode D90 is larger, the resistance component of diode D90 is lowered and the voltage drop is mitigated. It is therefore possible to reduce amplitude-amplitude distortion.
As described above, the amplitude-phase distortion characteristic and the amplitude-amplitude distortion characteristic have to be compensated for to lower distortion of the power amplifier. In the above described distortion compensation method in power amplifier 900, however, only the non-linearity of the resistance and capacitance components of diode D90 is used to provide compensation. Thus, once diode 90 to be used is determined, the non-linearity of the resistance and capacitance components are fixed at the same time. The non-linearity of them cannot be set to an optimum value separately.
Consequently, in some cases, only one of amplitude-phase distortion and amplitude-amplitude distortion can be compensated for or distortion compensation of one of them can exacerbate the other distortion. When the amplitude-phase distortion and amplitude-amplitude distortion are to be compensated for at the same time, compensation for both may be insufficient.
Such a power amplifier has to be operated at a higher bias current to maintain low distortion. If low distortion is realized, the power efficiency decreases, that is, the power consumption increases. Therefore, when the power amplifier is used for a battery-driven communication terminal, the communication time till the battery runs out is shortened.
Therefore, the present invention provides a power amplifier of low distortion and high power efficiency, and a low power consumption communication device including the power amplifier.
A power amplifier according to one aspect of the present invention includes a power amplification element including a first bipolar transistor of a common-emitter type, a voltage supply circuit for supplying the base of the first bipolar transistor with a bias voltage, and a distortion compensation circuit for compensating for distortion of the power amplification element. The distortion compensation circuit includes a variable impedance element provided between the voltage supply circuit and the base of the first bipolar transistor, and an adjustment circuit for adjusting at least one of a reactance component and a resistance component from the first transistor toward the variable impedance element.
Therefore, according to the power amplifier, the reactance component and the resistance component from the base end of the bipolar transistor for amplification toward the variable impedance element can be separately adjusted. Thus, amplitude-amplitude distortion and amplitudephase distortion can be separately compensated for. As a result, lower distortion of the power amplifier can be realized.
Preferably, the power amplifier includes a resistor having one terminal connected to the power supply circuit, and a capacitor connected between the other terminal of the resistor and a ground potential.
Therefore, according to the power amplifier, the reactance component and the resistance component from the base end of the bipolar transistor for amplification toward the variable impedance element can be separately adjusted by the resistor and the capacitor. Particularly, the smaller the resistance component is, the smaller the gain of the power amplifier becomes because of signal power consumption with the resistance component. For example, when adjustment of the reactance component is more effective than adjustment of the resistance component for distortion compensation, however, addition of the resistor can increase the resistance component and improve the gain of the power amplifier.
More preferably, the variable impedance element is formed of a diode element having an anode connected to the voltage supply circuit and a cathode connected to the base of the first bipolar transistor.
Therefore, according to the power amplifier, a diode is particularly used as the variable impedance element, and therefore the both end voltage dependence of the variable impedance element comes to have the same type as the both end voltage dependence of impedance at a diode portion between the base and emitter of the bipolar transistor used for amplification. It can be especially effective for distortion compensation in the case of a wide-ranging input power.
More preferably, the variable impedance element includes a second bipolar transistor configured to form a PN junction between the voltage supply circuit and the base of the first bipolar transistor.
Therefore, according to the power amplifier, employment of the diode portion of the bipolar transistor as the variable impedance element enables the variable impedance element to be manufactured in the same process as the bipolar transistor for amplification. Since a semiconductor element used for the power amplifier can be limited to one type of bipolar transistors, it simplifies the device parameter extraction process for circuit design of circuit elements used for the power amplifier. Since a power amplifier circuit including the variable impedance element can be formed in a monolithic manner on a semiconductor substrate, the power amplifier can be miniaturized.
More preferably, the variable impedance element is formed of a second bipolar transistor having an emitter connected to the base of the first bipolar transistor, a base connected to the voltage supply circuit, and a collector connected to a node for connecting the resistor and the capacitor.
Therefore, according to the power amplifier, the emitter current of the second bipolar transistor is a sum of the base current and the collector current. Since the collector current is almost proportional to the base current, the emitter current also has a diode-like current-voltage characteristic. Therefore, the second bipolar transistor functions as a variable impedance element.
Since the collector current is made variable by the resistor as a result, the emitter current is also made variable by the resistor. Therefore, even after a bipolar transistor, which also serves as a variable impedance element to be used, is selected, the variable resistance characteristic of the bipolar transistor can be adjusted by the resistor. As a result, the freedom of adjusting distortion compensation is increased.
More preferably, the variable impedance element is formed of a second bipolar transistor having a collector connected to the base of the first bipolar transistor, a base connected to the voltage supply circuit, and an emitter connected to a node for connecting the resistor and the capacitor.
Therefore, according to the power amplifier, the collector current of the second bipolar transistor has a diode-like current-voltage characteristic for the bias voltage and functions as a variable impedance element. In this case, since the emitter current is made variable by the resistor, the collector current is also made variable by the resistor. Therefore, even after a second bipolar transistor to be used is selected, the variable resistance characteristic of the second bipolar transistor can be adjusted by the resistor, and the freedom of adjusting distortion compensation is increased.
Preferably, the adjustment circuit includes a resistor having one terminal connected to the voltage supply circuit and the other terminal connected to the variable impedance element, and a capacitor connected between the voltage supply circuit and a ground potential.
Therefore, according to the power amplifier, the reactance component and the resistance component from the base end of the bipolar transistor for amplification toward the variable impedance element can be separately adjusted by the resistor and the capacitor. Thus, both amplitude-amplitude distortion and amplitude-phase distortion can be compensated for and the amplifier can have lower distortion.
More preferably, the variable impedance elements is formed of a second bipolar transistor having an emitter connected to the base of the first bipolar transistor, a collector connected to the voltage supply circuit, and a base connected to the other terminal of the resistor.
Therefore, according to the power amplifier, the emitter current of the second bipolar transistor is a sum of the base current and the collector current. Since the collector current is almost proportional to the base current, the emitter current also has a diode-like current-voltage characteristic for the bias voltage. Therefore, the second bipolar transistor functions as a variable impedance element. Since the base current is made variable by the resistor as a result, the collector current and the emitter current are also made variable by the resistor. Thus, even after a second bipolar transistor to be used is selected, the variable resistance characteristic of the second bipolar transistor portion can be adjusted by the resistor, and the freedom of adjusting distortion compensation is increased.
More preferably, the variable impedance element is formed of a second bipolar transistor having a collector connected to the base of the first bipolar transistor, an emitter connected to the voltage supply circuit, and a base connected to the other terminal of the resistor.
Therefore, according to the power amplifier, the collector current in the second bipolar transistor has a diode-like current-voltage characteristic for the bias voltage. Therefore, the second bipolar transistor functions as a variable impedance element. Since the base current is made variable by the resistor in this case, the emitter current and the collector current are also made variable by the resistor. Thus, even after a second bipolar transistor to be used is selected, the variable resistance characteristic of the second bipolar transistor portion can be adjusted by the resistor, and the freedom of adjusting distortion compensation is increased.
More preferably, the first bipolar transistor and the variable impedance element are formed on one semiconductor substrate.
Therefore, according to the power amplifier, the bipolar transistor for amplification and the variable impedance element which are formed on one substrate can be configured in a monolithic manner. Therefore, the power amplifier itself can be miniaturized.
Furthermore, the variable impedance element can be manufactured in the same process as the bipolar transistor for amplification. Since a semiconductor element used for the power amplifier can be limited to one type of bipolar transistors, it is possible to simplify the device parameter extraction process for circuit design of circuit elements used for the power amplifier.
Preferably, the first bipolar transistor operates in a Class B or Class AB mode.
Therefore, according to the power amplifier, employment of the variable impedance element and the adjustment circuit enables compensation of amplitude-amplitude distortion and amplitude-phase distortion. Thus, the bipolar transistor for amplification can be operated at a bias current in about the Class B or Class AB operating mode. As a result, the efficiency of the linear amplifier can be improved.
Preferably, the gain of the power amplifier can be controlled by controlling an output voltage of the voltage supply circuit.
Therefore, according to the power amplifier, the distortion compensation circuit is also used as the bias circuit of the first bipolar transistor. By controlling the output voltage of the voltage supply circuit, therefore, the bias current of the first bipolar transistor can be controlled. Thus, the gain of the power amplifier can be controlled while the function of distortion compensation is maintained. It is therefore possible to improve the power efficiency of a power amplifier used especially in a communication system which requires low distortion of the power amplifier and gain control in a wide dynamic range such as W-CDMA (Wide Band-Code Division Multiple Access) and IS-95 (interim standard 95).
Preferably, the power amplifier controls the distortion compensation amount of the distortion compensation circuit by controlling the output voltage from the voltage supply circuit.
Therefore, according to the power amplifier, a DC voltage across the variable impedance element included in the distortion compensation circuit can be controlled by controlling the output voltage of the voltage supply circuit. Thus, the impedance of the variable impedance element can be controlled. Since the distortion compensation amount of the distortion compensation circuit can be adjusted as a result, it is possible to provide distortion compensation corresponding to the degree of distortion caused in the first bipolar transistor. As a result, the freedom of distortion compensation is increased as compared with a case where the output voltage of the voltage supply circuit is fixed.
A communication device according to a further aspect of the present invention includes a power amplifier having a power amplification element including a first bipolar transistor of a common-emitter type for signal amplification and a distortion compensation circuit for compensating for distortion of the power amplification element, and a voltage supply circuit for supplying the base of the first bipolar transistor with a bias voltage. The distortion compensation circuit includes a variable impedance element provided between the voltage supply circuit and the base of the first bipolar transistor, and an adjustment circuit for adjusting at least one of a reactance component and a resistance component from the first bipolar transistor toward the variable impedance element.
Preferably, the adjustment circuit includes a resistor having one terminal connected to the voltage supply circuit, and a capacitor connected between the other terminal of the resistor and a ground potential.
More preferably, the variable impedance element is formed of a diode element having an anode connected to the voltage supply circuit and a cathode connected to the base of the first bipolar transistor.
More preferably, the variable impedance element includes a second bipolar transistor configured to form a PN junction between the voltage supply circuit and the base of the first bipolar transistor.
More preferably, the variable impedance element is formed of a second bipolar transistor having an emitter connected to the base of the first bipolar transistor, a base connected to the voltage supply circuit, and a collector connected to a node for connecting the resistor and the capacitor.
More preferably, the variable impedance element is formed of a second bipolar transistor having a collector connected to the base of the first bipolar transistor, a base connected to the voltage supply circuit, and an emitter connected to a node for connecting the resistor and the capacitor.
Preferably, the adjustment circuit includes a resistor having one terminal connected to the voltage supply circuit and the other terminal connected to the variable impedance element, and a capacitor connected between the voltage supply circuit and a ground potential.
More preferably, the variable impedance element is formed of a second bipolar transistor having an emitter connected to the base of the first bipolar transistor, a collector connected to the voltage supply circuit, and a base connected to the other terminal of the resistor.
More preferably, the variable impedance element is formed of a second bipolar transistor having a collector connected to the base of the first bipolar transistor, an emitter connected to the voltage supply circuit, and a base connected to the other terminal of the resistor.
More preferably, the first bipolar transistor and the variable impedance element are formed on one semiconductor substrate.
Preferably, the first bipolar transistor operates in a Class B or Class AB mode.
Preferably, the gain of the power amplifier is controlled by controlling an output voltage of the voltage supply circuit.
Preferably, the distortion compensation amount of the distortion compensation circuit is controlled by controlling the output voltage of the voltage supply circuit.
Therefore, according to the communication device, the power amplifier for transmission has low distortion and high efficiency and thus the power consumption of the communication device is reduced. Especially for a battery-run communication device, the communication time till the battery runs out can be increased. For attaining the same communication time as conventional products, a much smaller battery can be employed, resulting in a smaller or lighter communication terminal.
Preferably, the communication device is used in a communication system in which a signal includes an amplitude modulation component. If a transmitted signal includes an amplitude modulation component, distortion of the waveform of the transmitted signal at an amplification stage for amplifying the transmitted signal to a predetermined antenna output level makes it impossible to correctly demodulate the transmitted information at the receiver side. Therefore, the communication system requires a low distortion power amplifier which faithfully amplifies and outputs an input signal waveform as the power amplifier for transmission power. Such a communication system is, for example, W-CDMA, IS-95, PDC (Personal Digital Cellular), PHS (Personal Handy-Phone System), IMT-2000 (International Mobile Telecommunication 2000) and a wireless LAN (Local Area Network) at 5 GHz band.
Therefore, according to the communication device, the low distortion power amplifier is included for transmission and thus correct information can be transmitted to the receiver side without distortion of the waveform of the transmitted signal.
If the above-described communication device is used for the communication system such as W-CDMA, IS-95, PDC, PHS and IMT-2000 which requires a severe low distortion characteristic represented by adjacent channel leakage power standard for a power amplifier for transmission, it is possible to attain both low distortion and high efficiency. Since the power consumption of the communication device can be reduced, the communication time till the battery runs out can be increased if the communication device runs on a battery. For attaining the same communication time as conventional product, a much smaller battery can be used, resulting in a smaller or lighter communication terminal.
Preferably, the communication device further includes a detection circuit for detecting a signal power level input to the power amplifier or a signal power level output from the power amplifier, and a control circuit for controlling an output voltage of the voltage supply circuit according to the signal power level detected by the detection circuit.
Therefore, according to the communication device, even in a communication system which requires signal amplification with low distortion, the gain or the distortion compensation amount of the power amplifier can be controlled by controlling the output voltage of the voltage supply circuit according to the detected input signal level or output signal level so that power consumption is minimized with a prescribed gain or with distortion within a determined value. Therefore, the power consumption of the communication device is reduced and, in the case of a battery-driven communication device, the communication time till the battery runs out can be increased. For attaining the same communication time as conventional products, a much smaller battery can be used, resulting in a smaller or lighter communication terminal.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.